The endotoxin level of all SP-D preparations was 0.1–0.5 EU/ml (Limulus Lysate Assay, Cambrex, Walkersville, MD, USA). The CL-46 NCRD was prepared in Pichia pistoris as described [23]. Briefly, the alpha-helical coiled-coil neck region and the CRD of CL-46 was amplified by PCR and ligated into the pPIC9K-vector (Invitrogen; Carlsbad, CA, USA). The pPIC9K derivatives were transformed into XL-10 E. coli, purified, linearized and transformed into Pichia pastoris (GS115). Clones were double-selected by growth on histidine deficient plates and

plates with increasing concentrations of geneticin. Monoclonal antibodies.  mAb 245-01 and 245-02 and 246-02 through 246-08 were raised against SP-D by inoculating mice with 10 μg/ml of human SP-D as previously described BIBW2992 [24]. The 3C3-C-20 mAb was developed by Dr. Jeffrey Whitsett, Cincinnati Children’s Hospital and Medical Center, Cincinnati, OH. mAb 6B2, 7A10

and 7C6 were produced by Dr. Kuroki as described [25]. Binding of mAb to SP-D or NCRD.  SP-D preparations were diluted in coating buffer to a concentration of 2 μg/ml and coated on ELISA plates overnight, followed by washing and addition of mAb. The final concentration of mAb used for the ELISA was 1 μg/ml. Bound mAb were detected with HRP-conjugated donkey-anti mouse antibodies labelled followed by 3,3′,5,5′-tetramethylbenzidine peroxidase. OD450 values were measured on a POLARstar OPTIMA plate reader (BMG Labtech, Durham, NC, USA). Binding of NCRD to IAV or mannan.  Binding of NCRD fusion proteins to IAV or mannan was measured as described by use of the S-protein-binding site on the fusion tag of the NCRD. In brief, IAV (Phil82 DAPT chemical structure strain) or mannan was coated onto the surface of ELISA plates

and, following washing, NCRD were added [21]. After incubation and washing, S-protein HRP was added and peroxidase activity measured. Hemagglutination (HA) inhibition assay.  HA inhibition was measured by serially diluting collectins or other host defence protein preparations in round bottom 96-well plates (Serocluster U-Vinyl plates; Costar, Cambridge, MA, USA) using PBS containing calcium and magnesium as a diluent [26]. After adding 25 μl of IAV, giving a final concentration of 40 HA units per ml or 4 HA units/well, the IAV/protein mixture was incubated for 15 min at room temperature, followed by addition of 50 μl of a type Plasmin O human erythrocyte suspension. The minimum concentration of protein required to fully inhibit the hemagglutinating activity of the viral suspension was determined by noting the highest dilution of protein that still inhibited hemagglutination. Inhibition of HA activity in a given well is demonstrated by absence of formation of an erythrocyte pellet. If no inhibition of HA activity was observed at the highest protein concentration used, then the value is expressed as greater than the maximal protein concentration. Fluorescent focus assay of IAV infectivity.